1. Field of the Invention
The present invention relates to a novel optical element capable of detecting a smaller mark than an optical spot, an optical head using the optical element and a signal reproducing method using the optical head.
2. Description of the Related Art
In the field of optical recording, it has been demanded to record a signal in a higher density. To meet such a demand, various recording and reproducing methods have been proposed.
For example, in the technical field of the optical heads which read a signal by illuminating an optical recording medium with a reading light, it has been proposed to use a solid immersion lens (will be referred to as “SIL” hereunder) or a solid immersion mirror (will be referred to as “SIM” hereunder) as an optical element in order to write and/or read a signal in a high density, and a fringing light in a near field to read a signal with a rather higher numerical aperture (NA) than ever.
On the other hand, it has been proposed to dispose a metal needle at the end of an interference microscope in order to measure the shape of an object under examination.
The metal needle is sharpened to have a point of about 100 nm. This sharp end will have an electromagnetic interaction with a very thin coating of Cr on the surface of the object under examination. The magnitude of the interaction depends upon the distance between the needle point and Cr coating.
A light converged to the needle point will have the wave phase thereof shifted due to the above electromagnetic interaction. The wave shift is on the order of 10−8/√{overscore ( )}Hz but can be detected using the phase difference interference between the needle point and a light from a remote location (refer to “Apertureless Near-field Optical Microscope” F. Zenhausern, M. P. O' Boyle and H. K. Wickeramasinghe, Appl. Phys. Lett. 65(13), Sep. 2, 1994, and “Optical Data Storage Read Out at 256 Gbits/sq. in.” Y. Martin, S. Rshton and H. K. Wickeramasinghe, Appl. Phys. Lett. 71(1), Jul. 7, 1997).
It is expectable that the above principle permits to detect a signal with a higher resolution than by the SIL and SIM. However, the principle has not yet been used in practice in the field of optical recording. For example, the cantilever has to always be at a constant distance from the surface of an optical disc to effectively implement the principle in the technique disclosed in the above documents, the optical disc is illuminated with a laser light other than the reading light from behind the cantilever and a servo control is made according to the movement of the laser light to control the distance between the needle point and disc to be tens of nm. However, since the cantilever is moved slowly, no high-speed reading is possible.